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Lecture 4 1 BES 108D Organisms in their environment By Dr. Benazir Alam Copyright © 2025 Pearson Canada, Inc. 27 - 1  2 Chapter 25 The...

Lecture 4 1 BES 108D Organisms in their environment By Dr. Benazir Alam Copyright © 2025 Pearson Canada, Inc. 27 - 1  2 Chapter 25 The History of Life on Earth Continued…….. Copyright © 2025 Pearson Canada, Inc. 27 - 2 Over the last 500 million years, extinction rates have tended to increase when global temperatures were high Copyright © 2025 Pearson Canada, Inc. 27 - 3 Consequences of Mass Extinctions Mass extinction can alter ecological communities and niches available to organisms It can take from 5 to 100 million years for diversity to recover after a mass extinction Mass extinctions can change the types of organisms found in communities. Lineages with novel and advantageous features can be lost during mass extinctions By eliminating so many species, mass extinctions can pave the way for adaptive radiations where the survivors evolve rapidly to fill the vacant roles left by the extinct species, often leading to increased biodiversity. Copyright © 2025 Pearson Canada, Inc. 27 - 4 Worldwide Adaptive Radiations Mammals underwent an adaptive radiation after extinction of dinosaurs about 65 mya (shown by red dotted line) With the disappearance of the dinosaurs (except for birds), mammals expanded greatly in both diversity and size, filling the ecological roles once occupied by terrestrial dinosaurs Copyright © 2025 Pearson Canada, Inc. 27 - 5 Major Changes in Body form can Result from changes in the rate and timing of Developmental processes Heterochrony refers to evolutionary changes in the timing or rate of developmental processes, which can significantly influence the shape and structure of organisms. In the case of human and chimpanzee skulls, heterochrony plays a key role in the differences observed between the two species. Copyright © 2025 Pearson Canada, Inc. 27 - 6 Relative Skull Growth Rates q In the human evolutionary lineage, mutations slowed the growth of the jaw relative to other parts of the skull. As a result, in humans the skull of an adult is more similar to the skull of an infant than is the case for chimpanzees q While fetal skulls may appear similar, the subsequent developmental changes driven by heterochrony lead to the significant morphological differences observed in adult humans and chimpanzees. Copyright © 2025 Pearson Canada, Inc. 27 - 7 Changes in Gene sequence New morphological forms likely come from gene duplication events that likely produces novel morphological forms Differences in the expression and regulatory mechanisms of a specific Hox (Homeobox) gene sequence called Ubx (Ultrabithorax), contribute to the evolutionary transition from a more complex appendage pattern in crustaceans (more than 6 legs) to the more specialized six-legged form in insects. particular changes in the nucleotide sequence of a developmental gene contributed to a major evolutionary change: the origin of the six- legged insect body plan These changes in nucleotide changes might alter the coding region or regulatory elements that control gene expression. Copyright © 2025 Pearson Canada, Inc. 27 - 8 Effects of the Ubx, Ultrabithorax gene on the insect body plan This suppression is crucial for defining the distinct It allows for the maintenance of body plan of insects, where each segment has leg development across various specialized appendages segments. Copyright © 2025 Pearson Canada, Inc. 27 - 9 Evolution is not Goal Oriented Because evolution is not goal-oriented, it can lead to a wide diversity of forms and functions. Different populations may evolve in entirely different directions based on their unique circumstances, even if they share a common ancestor. “Evolution is like tinkering” --This analogy suggests that evolution does not design organisms from scratch but rather modifies existing structures and functions in small, incremental ways. Randomness of Mutation Natural Selection Adaptation to Current Conditions Copyright © 2025 Pearson Canada, Inc. 27 - 10  11 Topic:1 Prokaryotes: Bacteria and Archaea Chapter: 27 Copyright © 2025 Pearson Canada, Inc. 27 - 11 Copyright © 2025 Pearson Canada, Inc. 27 - 12  13 Prokaryotes Ø Microscopic single celled/Unicellular organisms which have neither a well-defined nucleus with a nuclear membrane or any other well-defined organelles. Ø Most prokaryotic cells are 0.5–5 µm (micron) in size, much smaller than the size of many eukaryotic cells which ranges between 10–100 µm. Ø Exception: Thiomargarita namibiensis ~ 70 µm, a gram-negative bacteria found in the ocean sediments of the continental shelf of Namibia Ø Prokaryotes are divided into two domains: bacteria and archaea Ø Prokaryotes thrive almost everywhere, including places too acidic, salty, cold, or hot for most other organisms Ø They can divide quickly, a feature that that gives them high genetic diversity Ø There are more prokaryotes in a handful of fertile soil than the number of people who have ever lived Copyright © 2025 Pearson Canada, Inc. 27 - 13  14 The most common shapes of prokaryotes Copyright © 2025 Pearson Canada, Inc. 27 - 14 Cell-Surface Structures Nearly all prokaryotic cells have a cell wall – It maintains cell shape, protects the cell, and prevents it from bursting in a hypotonic environment Bacterial cell walls contain peptidoglycan, a network of sugar polymers cross-linked by polypeptides Archaea contain polysaccharides and proteins but lack peptidoglycan In contrast, eukaryote cell walls are made of cellulose or chitin Copyright © 2025 Pearson Canada, Inc. 27 - 15 Gram staining Gram stain can be used to classify bacteria by cell wall composition. Crystal Violet+Iodine-Wash with ethanol- counter stain with saffranin. Gram-positive bacteria have simpler walls with a large amount of peptidoglycan that prevents the Crystal Violet+Iodine complex to move out by ethanol wash. It results in the crystal violet masking the red safranin dye. Gram-negative bacteria have less peptidoglycan and an outer membrane. During Gram staining, the crystal-violet- iodine complex can pass through this thin cell wall and is removed by ethanol wash. It results in the safranin dye staining the cell pink or red. Copyright © 2025 Pearson Canada, Inc. 27 - 16  Gram Staining q Many antibiotics such as penicillin target peptidoglycan crosslinking and damage bacterial cell walls q Gram-negative bacteria are more likely to be antibiotic resistant because the outer membrane impedes the entry to drugs Copyright © 2025 Pearson Canada, Inc. 27 - 17  Cell-Surface Structures: Capsule A dense and well defined polysaccharide or protein layer called a capsule covers many prokaryotes. Sometimes a not-so-well organized slime layer is present instead of a capsule. These structures enable prokaryotes to stick to substrate or other individual in a colony. They also protect against dehydration and prevent some pathogenic prokaryotes to be detected by the host immune system. Copyright © 2025 Pearson Canada, Inc. 27 - 18 Cell-Surface Structures: Fimbriae Some prokaryotes have hair like appendages on their surface called fimbriae, which allow them to stick to their substrate or other individuals in a colony Neisseria gonorrhoeae the causative agent of gonorrhea uses fimbriae to attach itself to mucus membranes. Do you expect a strain of Neisseria gonorrhoeae that does not have fimbriae be able to cause disease? Copyright © 2025 Pearson Canada, Inc. 27 - 19  Cell surface structure: Endospores Many prokaryotes form metabolically inactive endospores, which can remain viable in harsh conditions for centuries and reactivate when the conditions are favourable. An endospore is a dormant, tough, and non-reproductive structure produced by some bacteria Endospore formation is usually triggered by a lack of nutrients, and usually occurs in gram-positive bacteria. The original cell produces a copy of its chromosome and surrounds it with a tough multilayered structure, forming the endospore. Water is removed from the endospore, and its metabolism halts, preventing reproduction, development, and repair. The original cell then lyses, releasing the endospore. Most endospores are so durable that they can survive in boiling water; killing them requires heating lab equipment to 121°C under high pressure. Copyright © 2025 Pearson Canada, Inc. 27 - 20  Think Clostridium species make spores Why do you think Clostridium difficile infections in hospitals are of great concern? Copyright © 2025 Pearson Canada, Inc. 27 - 21  22 Endospores Bacillus anthracis, the bacterium that causes the disease anthrax in livestock and humans, produces endospores, that can remain dormant in soil for decades and become reactive upon inhalation or ingestion. Copyright © 2025 Pearson Canada, Inc. 27 - 22  Cell surface structure: Flagella (Salmonella typhimurium) Bacterial flagella are long, whip-like appendages that enable bacteria to move through their environment, a process known as motility. Flagella are rotating in a helical manner Copyright © 2025 Pearson Canada, Inc. 27 - 23 Motility In a heterogeneous environment, many bacteria exhibit taxis, the ability to move toward or away from a stimulus. Most motile bacteria propel themselves by flagella scattered about the surface or concentrated at one or both ends of the cell Chemotaxis is the movement toward or away from a chemical stimulus. Positive: Towards Negative: Away from a nutrients, oxygen or light toxic substance Copyright © 2025 Pearson Canada, Inc. 27 - 24 Internal Organization and DNA Prokaryotic genomes have less DNA than eukaryotic genomes and are associated with relatively fewer proteins Most of the genome consists of a single circular chromosome The chromosome is not surrounded by a membrane; it is located in the nucleoid region Some species of bacteria also have smaller circular DNA molecules called plasmids Copyright © 2025 Pearson Canada, Inc. 27 - 25 A Prokaryotic Chromosome and Plasmids Identify an additional plasmid visible in this micrograph? Copyright © 2025 Pearson Canada, Inc. 27 - 26 Internal Organization and DNA (3 of 3) There are some differences between prokaryotes and eukaryotes in DNA replication, transcription, and translation Smaller size of ribosomes (that are involved in protein translation) in prokaryotes allows antibiotics such as erythromycin (binds 23S ribosomal subunit) and tetracyclin (binds 30S ribosomal subunit) to inhibit translation and prevent bacterial growth without harming eukaryotic cells. Copyright © 2025 Pearson Canada, Inc. 27 - 27